The sundial should be horizontal with the thread oriented toward the Pole Star, as indicated by a compass and knowing the Pole Star altitude where the observation is to be performed. By means of such a device it is possible to know the time in a wide range of latitudes, so that in the past it was particularly useful for voyagers. When going North or South, one should orient accordingly the thread, which serves as gnomon, so that it still points towards the Pole Star.
The compass endowing the sundial helps to perform this operation even by the light of the day because it allows to find the north direction. Then the latitude of the main European cities (indicated on the quadrant perimeter) allows orienting the thread at the same angle with respect to the horizontal plane. With these two indications it is possible to find the Pole Star direction, since the star is situated on the meridian plane of the place, with angular altitude equal to the one of the observation point.
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